1. Field of the Invention
The present invention relates to a method of producing polymers having their aromatic rings hydrogenated (nuclear-hydrogenated), comprising a step of hydrogenating an aromatic vinyl compound—(meth)acrylate copolymer in a mixed solvent of an ester compound and an alcohol compound in the presence of a catalyst.
2. Description of the Prior Art
Non-crystalline plastics such as acrylic resins, methacrylic resins, styrene resins, polycarbonate resins and cyclic polyolefin resins have now been used in various application fields, and particularly have found increasing use as optical materials such as optical lenses and substrates for optical discs because of their excellent optical properties. Such optical materials are required to have, in addition to a high transparency, high functional properties well-balanced in a high heat resistance, a low water absorption and mechanical properties.
Known plastics do not necessarily meet these requirements and involve own problems to be solved. For example, polystyrene is mechanically brittle, large in birefringence and poor in transparency. Polycarbonate is excellent in heat resistance, but has a large birefringence and a transparency as poor as polystyrene. Polymethyl methacrylate is highly transparent, but poor in dimension stability because of extremely high water absorption and low in heat resistance. Polyvinylcyclohexane which is produced by the nuclear hydrogenation of polystyrene is excellent in transparency, but has a low mechanical strength, a poor heat resistance and a poor adhesion to other materials (for example, JP 2003-1308078A, Japanese Patent 3094555, and JP 2004-149549A).
Copolymers of methyl methacrylate (MMA) and styrene (MS resin) are highly transparency and well balanced in dimension stability, rigidity, specific gravity, etc., but exhibit a large birefringence.
Nuclear-hydrogenated MS resins (MSH resins), particularly MSH resins having a MMA unit content of 50 mol % or more exhibit, as compared with MS resins, a birefringence extremely lowered and are known to be well balanced in transparency, heat resistance and mechanical properties.
The nuclear hydrogenation of aromatic polymers are already known. It has been recognized in the art that the degree of nuclear hydrogenation must be increased for attaining a high transparency, and therefore, highly transparent resins cannot be obtained unless the degree of nuclear hydrogenation is increased to about 100%. This is because that the resultant polymer has a block structure when the degree of nuclear hydrogenation is low, to lower the total light transmittance. Aromatic polymers are not easily nuclear-hydrogenated because of their high molecular weights. Therefore, it has been proposed to design the micro pore structure of catalyst (for example, JP 11-504959T). However, it is difficult to reach 100% degree of nuclear hydrogenation. Therefore, it has been demanded to provide a method which is capable of attaining a high transparency even when the degree of nuclear hydrogenation is lower.
The nuclear hydrogenation is largely affected by the solvent because it is a reaction of macromolecules. Various solvents such as hydrocarbons, alcohols, ethers and esters are hitherto used for the nuclear hydrogenation. However, these solvents involve problems: hydrocarbons and alcohols are poor in dissolving power to aromatic polymers; ethers, for example, 1,4-dioxane has a low ignition point; and tetrahydrofuran is instable because it is easily subject to ring-opening reaction; and esters make the resultant polymers cloudy depending on the degree of nuclear hydrogenation. Thus, there has been proposed no safe and stable method capable of quickly producing highly transparent nuclear-hydrogenated aromatic polymers. It has been reported that a high transparency can be attained even at a low degree of nuclear hydrogenation by adding alcohol or water to ether solvents (for example, Japanese Patent 2890748). However, the nuclear-hydrogenated polymer produced by the proposed method fails to satisfy the high transparency required for optical materials.